One technique for doping silicon wafers is to direct a beam of ions to impinge upon a wafer to produce controlled concentrations of doping impurities within the wafer.
In medium and low current ion implanters an ion beam is directed across a wafer surface by x-y deflection scanning of the beam in a raster or similar pattern. This has been done using two orthogonal pairs of electrostatic deflection plates to produce the beam deflections. Application of triangular waveform voltages to the deflection plates can produce rectangular raster scan patterns on the wafer. U.S. Pat. No. 4,514,637 to Dykstra et al. discloses one such medium to low current ion implanter. The disclosure of the Dykstra et al. patent is incorporated herein by reference.
Traditional deflection systems use a high frequency scan in one direction and a low frequency scan in an orthogonal direction to sweep across a circular semiconductor wafer. The relationship between the low and high scan frequencies is selected to produce a highly interlaced lissajous pattern. The scan pattern produced at the target is rectangular or square and includes portions that overscan the circular wafer.
Prior art ion beam scanning systems utilize visual displays that aid in focusing the ion beam and in centering the ion beam scanning pattern about the workpiece. These prior art displays use signals proportional to ion beam current to deflect a cathode ray tube display vertically as a horizontal sweep of the display is synchronized with the ion beam deflection voltages.
The square pattern generated by a combination of high and low frequency scan signals necessitates two visual displays for beam tuning and centering. One low frequency horizontal sweep on a first display is synchronized with the low frequency scan signal and a second high frequency horizontal sweep on a second display is synchronized with the high frequency scan signal. To achieve an optimum scan pattern with a single display monitor a user must switch back and forth between the two displays. Experience with this type of beam tuning indicates that the display may not accurately depict the shape and/or trajectory of the ion beam. If, for example, there is a malfunction that affects one display but not the other improper beam treatment might go unnoticed with adverse affect on dose uniformity and product yields from the doped semiconductor wafer.
If the user is to rely upon a visual display for beam focusing and centering, the display must accurately depict the beam scanning pattern. Prior art displays trigger horizontal sweeps of a cathode ray tube with the ion beam deflection voltages and deflect the CRT imaging ray vertically based upon the ion beam current. It is known, however, that it takes the ion beam a certain time to pass from the region of the deflection plates to the workpiece. To compensate for this time of flight lag, prior art systems delay the CRT sweep trigger a constant time period to coordinate the beam deflection with the beam current.